Dash pot oil
 

I know that the book recommends ATF or similar but what effect would it have on performance using a heavier, engine oil for example, or a lighter oil ?

a heavier oil would make the piston rise slower restricting power and increasing fuel consumption .

Stromberg carbs should use ATF

SU carbs should have SAE20 oil but I find 10w40 works well. Slightly thicker oil will give more enrichment is acceleration, something these engines tend to need.

That was my understanding. Thicker oil slows the rise of the piston under heavy acceleration and momentarily causes a richer mixture.

ATF is more stable across a range of temperatures

ATF is more stable, because it is single weight. Straight SAE20 weight oil is ideal really, in my experience, and as recommended by SU/Burlen: http://sucarb.co.uk/technical-carbur...gs-chamber-oil

I found some single 20w oil for much less however (I think it was for a lawnmower, can't remember).
10w40 etc would change characteristics as the engine warmed up.

I always used 3-in-1 oil as dashpot oil for a pair of SUs fitted on a 60s 997S engined mini - it was quoted as an SAE20 oil.

I will stick to ATF used in my cars including rally stuff and in my commercial repair business for 40 + years..but basically its personal choice.not wishing to get into a pi##ing competition. No offence to anyone as if you ask 100 people they will all give well intentioned advice

Experience always counts in these matters - as far as I'm concerned it always comes down to personal experience/expertise and what you know works for you and the machine in front of you at the time and how you want it to perform.

The biggest mistake I ever made was changing the SU needles for richer needles and using a lower 10SAE oil giving a faster drop/drop on pair - trying to balance them was a nightmare so eventually they replaced them with a pair of Webers on a smoothed & re-profiled inlet manifold, but that was a wholly different balancing act!

AS;

Don't take this wrong, but where do I start...based on what did you change Metering Needles and Damper oil viscosity?...if for any reason less than full range dynamometer motor tests, where engine response and exhaust gases are checked under all load (steady-state and transient) conditions, I expect that explains half of the tuning problems...

From: https://www.sw-em.com/su_carbs.htm
"...this is exactly why wrong Metering Needles get into carbs: ...he says: "...you can kinda look at the chart....I think I'm going to go to an XXX needle". What kind of a system for picking a needle is that? None! Metering Needles for a particular configuration of engine (mostly influenced by camshaft, manifolding - intake and exhaust flow) were arrived at after countless hours on a manufacturer's motor test stand (dynamometer), running at various RPMs, loads, throttle settings, and with exhaust gas analyzers... very few casual mechanics or even shops are equipped enough (mentally or hardware) to get this any better than the factory!...but they think they are!...so they'll try...and that's why incorrect needles are found in carbs! Do not change Metering Needles unless you can put motor/vehicle on a dynamometer to scientifically check the results...like so! (see Reference: B18 on dynamometer) ...and if you feel the need to change the mixture slightly (for instance seasonably), adjust the Mixture Adjustment Nut only...no more!"

...and Damper Oil should be 20 weight (and because that is what ATF is, and because it has a wide operating Temp range, and is available inexpensively in quarts, it is a popular alternative)...and BTW... Dashpot Drop is not dampened...only Rise!
https://www.sw-em.com/SU%20Damper%20Valve%20markup.jpg

I doesn't surprise me that you gave up in the end and went to Webers...I wonder how that went...

Cheers